Improve guidance on including os name

[thompson-tomo]

See confusion in #2996

However, to clarify — when we refer to avoiding OS names at the “root namespace” level, we also mean avoiding them at the area level. The OS name should appear after the area of concern (such as system.memory.linux.*), not before it. This ensures that users can first navigate by functional area (e.g. memory, CPU, network) and then, if necessary, drill down into OS-specific variants within that area.

Guidance should be added as what to do when an area is only applicable to linux or unix. Does it go before or after.

At the same time what is an area? For instance if memory is an area then shouldn't the memory attributes be in the memory namespace rather than system.memory?